In the oil and gas industry, geophysical prospecting techniques are commonly used to aid in the search for and evaluation of subterranean hydrocarbon deposits. Generally, a seismic energy source is used to generate a seismic signal which propagates into the earth and is at least partially reflected by subsurface seismic reflectors (i.e., interfaces between underground formations having different acoustic impedances). The reflections are recorded by seismic detectors located at or near the surface of the earth, in a body of water, or at known depths in boreholes, and the resulting seismic data may be processed to yield information relating to the location of the subsurface reflectors and the physical properties of the subsurface formations.
One type of geophysical prospecting utilizes an impulsive energy source, such as dynamite or a marine air gun, to generate the seismic signal. With an impulsive energy source, a large amount of energy is injected into the earth in a very short period of time. Accordingly, the resulting data generally have a relatively high signal-to-noise ratio, which facilitates subsequent data processing operations. On the other hand, use of an impulsive energy source can pose certain safety and environmental concerns.
In the late 1950s and early 1960s, Conoco Inc. pioneered development of a new type of geophysical prospecting, generally known as "vibroseis" prospecting. Vibroseis prospecting employs a land or marine seismic vibrator as the energy source. In contrast to an impulsive energy source, a seismic vibrator imparts a signal into the earth having a much lower energy level, but for a considerably longer period of time.
The seismic signal generated by a seismic vibrator is a controlled wavetrain (i.e., a sweep) which is applied to the surface of the earth or in the body of water. Typically, a sweep is a sinusoidal vibration of continuously varying frequency, increasing or decreasing monotonically within a given frequency range, which is applied during a sweep period lasting from 2 to 20 seconds or even more. The frequency may vary linearly or nonlinearly with time. Also, the frequency may begin low and increase with time (upsweep), or it may begin high and gradually decrease (downsweep).
The seismic data recorded during vibroseis prospecting (hereinafter referred to as "vibrator data") are composite signals, each consisting of many long, reflected wavetrains superimposed upon one another. Since these composite signals are typically many times longer than the interval between reflections, it is not possible to distinguish individual reflections from the recorded signal. Thus, as is well known to persons skilled in the art, one of the first steps in processing seismic vibrator data is to cross-correlate the recorded data with the sweep signal (also known as the "reference signal"). See, e.g., Kirk, P., "Vibroseis Processing," Chapter 2 of Developments in Geophysical Exploration Methods--2, edited by A. Fitch, Applied Science Publishers Ltd., London, 1981, pp. 37-52. The resulting correlated data approximate the data that would have been recorded if the source had been an impulsive energy source.
One undesirable byproduct of the conventional cross-correlation process is the existence of excessive amounts of side lobe energy in the resulting wavelet. Recently, a new type of sweep known as a "shaped" sweep has been developed to resolve this problem. The primary benefit of a shaped sweep is that the correlated vibrator data will have a simple wavelet shape and minimal side lobe energy. This is accomplished by shaping the sweep so as to yield a specific power spectrum, as more fully described in copending U.S. patent application Ser. No. 08/086,776 filed Jul. 1, 1993.
The amount of energy injected into the earth during a conventional vibrator sweep is governed by the size of the vibrator and the duration of the sweep. Given current practical limitations on both vibrator size and sweep duration, it is usually necessary to generate several sweeps at each source point. Each sweep is typically followed by a listen period during which the vibrator is not sweeping, but reflection energy is still being received by the seismic detectors. Data resulting from each sweep are then cross-correlated with the reference signal for that sweep, and the resulting individual data traces are summed or "stacked" to obtain the final composite data trace for the source point. A significant portion of the time required for each source point is associated with the listen time between sweeps. Obviously, the efficiency of vibroseis prospecting could be significantly improved by eliminating part or all of this listen time.
Another problem with conventional vibroseis prospecting results from the fact that vibrators generate harmonic distortion as a result of non-linear effects in the vibrator hydraulics and the ground's non-linear reaction to the force exerted by the vibrator base plate, with the second, third, and fourth harmonics accounting for most of the distortion. These harmonics are present in the recorded data and lead to trains of correlated noise, known as harmonic ghosts, in the correlated data. These harmonic ghosts are particularly troublesome in the case of downsweeps where they occur after the main correlation peak (i.e., positive lag times) and, therefore, can interfere with later, hence weaker, reflections. In the case of upsweeps, harmonic ghosts are somewhat less troublesome because they precede the main correlation peak (i.e., negative lag times). Nevertheless, harmonic ghosts can cause difficulties in processing and interpreting data from upsweeps as well as from downsweeps.
In a 1972 publication, S. Sorkin described a method for removing even-numbered harmonics from correlated data. See, Sorkin, S., "A Method for Reducing the Effects of Base Plate Distortion," presented at the 1972 Pacific Coast joint meeting of the Society of Exploration Geophysicists and the American Association of Petroleum Geologists, Bakersfield, Calif., Mar. 9 and 10, 1972. Sorkin's method exploits the fact that the final composite data trace is the algebraic sum of several individual data traces. In Sorkin's method, only half of the individual data traces are generated in the conventional manner. The other half are generated with a reversed polarity sweep. During the summation or stacking process, the polarity of the data from the second group is reversed at the input of the recording system so that the second group of data traces are identical to the first group. The even-numbered harmonics, however, are unaffected by this second polarity reversal, and therefore, the stacking process causes the even-numbered harmonics from the second group of traces to cancel those of the first group.
In 1981, E. Rietsch proposed a generalization of Sorkin's method which permits elimination of harmonics of a sweep up to any desired order. See, Rietsch, E., "Reduction of Harmonic Distortion in Vibratory Source Records," Geophysical Prospecting, v. 29, pp. 178-188, 1981. Rietsch's method requires that a series of M signals be used where each signal has an initial phase angle differing from that of the previous signal by the angle 2.pi./M. Prior to stacking, the individual data traces are correlated with their respective sweep signals. By using this method, all harmonics up to and including the Mth harmonic cancel. The (M+1)th harmonic is present in the correlated data, and the following M-1 harmonics cancel, and so on.
Other methods have also been proposed for suppressing correlation noise. In 1982, Edelmann and Werner proposed two possible methods for doing so. See, Edelmann, H. A. K. and Werner, H., "Combined Sweep Signals for Correlation Noise Suppression," Geophysical Prospecting, v. 30, pp. 786-812, 1982. Their first method, known as the "Combisweep" technique, consists of using two or more sequential conventional sweeps having different frequency ranges. Symmetric or asymmetric frequency weighting for the composite trace can be achieved by overlapping the frequency spectra of the different sweeps. Their second method, known as the "Encoded Sweep" technique, uses a number of short sweeps combined into code sequences without time gaps. Two of these code sequences, the code and the complementary code, with a listening period in between, form the final encoded sweep. These methods, however, were not intended to improve the efficiency of vibroseis prospecting, and in general were no more efficient than conventional vibrator techniques.
U.S. Pat. No. 4,823,326 issued Apr. 18, 1989, to R. M. Ward describes a seismic data acquisition technique that permits simultaneous use of two or more seismic vibrators located at different source locations. Each of the vibrators uses a sequence of at least four individual sweeps. The pilot signal for each vibrator has a plurality of separate phase angles, and the sweep sequence for each vibrator is different from that of the other vibrator(s). The recorded signals are correlated against each of the sweep sequences. This generates separate correlated records for each of the sweeps, which are then summed in the appropriate manner to separate the data originating from each source location. Ward utilizes the method of Rietsch, described above, to suppress harmonic ghosts. Ward, however, does not improve vibrator efficiency by eliminating part or all of the listen time between individual sweeps. Rather, any improvement in efficiency is due to the use of multiple vibrators.
In a 1990 paper, Ward et al. stated that concatenating sweeps with the proper phase coding would allow the elimination of the intervening listen times to enhance the efficiency of vibroseis prospecting. See, Ward, R. M., Brune, R. H., Ross, A., and Kumamoto, L. H., "Phase Encoding of Vibroseis Signals for Simultaneous Multisource Acquisition," presented at the Sixtieth Annual International Meeting & Exposition of the Society of Exploration Geophysicists, San Francisco, Calif., September 23-27, 1990. However, Ward et al. failed to note that concatenating sweeps together into a sequence produces harmonic ghosting at both negative- and positive-lag times when correlated, regardless of sweep direction. For this reason, current methods for concatenating or linking sweeps together require the use of pairs of vibrator sweep sequences in order to suppress harmonic ghosting. The sweeps in the second sequence, the complementary sequence, are phase rotated in the opposite direction from the first sequence. Further, for this technique to be effective, both sweep sequences must be vibrated at the same source point.
Obviously, a need exists for a method of concatenating or linking sweeps together so as to reduce or eliminate the unproductive listen time without requiring the use of pairs of vibrator sweep sequences in order to suppress harmonic ghosting. The present invention provides such a method.